CML (chronic myeloid leukemia) is unique in that the BCR/ABL oncogene drives disease onset and progression. Therapies inhibiting the BCR/ABL kinase, such as imatinib mesylate, have dramatically altered the treatment of chronic phase disease, but are less effective in blast crisis and against specific point mutations in BCR/ABL. Blast crisis is a very aggressive phase of the disease characterized by genomic instability and secondary mutations. These features have been linked to DNA damage caused by reactive oxygen species (ROS). We and others find that overexpression of p210 BCR/ABL alone causes an increase in levels of ROS. Our preliminary data reveal that a consequence of this increased level of ROS is upregulation of Fyn, a Src kinase family member. We hypothesize that ROS dependent signaling transduced through Fyn contributes to progression to blast crisis. BCR/ABL overexpressing cells display a four-fold upregulation of Fyn protein. This upregulation is blocked by antioxidants, thus linking increased ROS levels and Fyn expression. Point mutations in BCR/ABL that confer resistance to imatinib mesylate retain upregulation of Fyn, suggesting that Fyn may be an appropriate therapeutic target in refractory patients. Knockdown of Fyn using shRNA slows leukemia cell growth in vitro and in vivo, inhibits clonogenic growth by 45% and causes increased sensitivity to imatinib. These striking changes indicate that Fyn is important for leukemia cell proliferation and sensitivity to imatinib. Blockade of Fyn also lowers the overall levels of intracellular peroxides, suggesting that Fyn can amplify ROS levels, thus contributing to subsequent DNA damage and secondary mutations. These preliminary data place Fyn both downstream and upstream of increased ROS, suggesting the presence of a positive feedback loop, whereby Fyn activation through BCR/ABL leads to increased ROS potentially playing an important role in blast crisis. This proposal will elucidate the mechanism(s) and consequences of oxidant-dependent Fyn upregulation and validate these findings in murine models and in clinical specimens. The specific aims of this proposal are to: #1: Determine the mechanism by which BCR/ABL and ROS levels upregulate Fyn. # 2: Evaluate the effects of Fyn upregulation on ROS dependent proliferative signals and DNA damage. #3: Test the relative contributions of BCR/ABL initiated ROS and Fyn upregulation on progression in CML murine models and patient specimens. These data will provide insight into one significant consequence of BCR/ABL-mediated ROS alterations: Fyn upregulation. Our long-term goal is to understand oxidant-dependent signaling in leukemia in order to devise strategies to prevent progression. Tactics that quelch ROS production, or block downstream signaling through Fyn in BCR/ABL containing cells may provide promising therapeutic modalities.